I wanted to write about nightmares, as many parents presume that is what is causing issues in their little one's sleep. In most cases children under 2, are too young to be experiencing such a thing, so what is a nightmare? and how can you tell, that is what your child is going through?
Plain and simply, a nightmare is a dream - that can startle you awake, feeling full of dread and in young children, shaking and crying. Most nightmares start out as pleasant dreams, and then turn into something more sinister.
It might start out as something simple like looking at fish in an aquarium, then a shark comes long, shows you its teeth, opens its mouth and swallows you whole, you now feel as though can't breathe and wake up in a sweat, gasping for breath, it can sometimes even take a few minutes to realize, you are safe in your bed.
Dreams occur in REM sleep only and are known to occur in the second year of life and on. Younger children have a harder time understanding any kind of dream from reality, and may hold onto the fear of an object (such as a monster) once the dream is over, in essence the monster is not real, but the fear of the monster is.
As children develop and grow, the dreams become more complex, and by the age of 5, most children can wake up and rationalize that they have had a bad dream, and deal with the feelings associated with it.
Most dreams, good / bad will stem from your child's waking life and day to day activities or experiences, if you can get a verbal descriptions from your child, you can very likely link it back to something that happened that day.
You can't fully prevent nightmares, but you can help your child deal with them by being there and comforting them when they are frightened. Older children who can describe their dreams can talk it through with you, and you can remind them its just a dream, even though you need to be sympathetic towards the fear, which is real. Lots of children going through fearful stages, may request to have the light on or the door open, and this is perfectly reasonable, this gives them a feeling of being safe there.
One thing I suggest you never do is go looking for monsters with your child, commonly known as the 'monster sweep', if you do this, you are confirming that such a thing could exist, as you are looking for it.